There are various recognized methods for dyeing and/or incorporating additives into fibers and other fibrous materials, including methods which utilize organic solvents such as dimethylsulfoxide ("DMSO"), dimethylacetamide ("DMAc"), dimethylformamide ("DMF"), N-methylpyrrolidone ("NMP"), N-cyclohexyl-2-pyrrolidone ("CHP"), acetophenone, acetanilide and methyl benzoate. Exemplary systems utilizing these organic solvents have been proposed in U.S. Pat. Nos. 3,558,267 to Langenfeld, 3,741,719 to Ramanathan et al, 4,525,168 to Kelly, and U.S. Pat. No. 4,898,596 to Riggins et al and U.K. Patent Nos. 1,275,459 to Gruen and 1,282,113 to Burtonshaw et al.
The organic solvents are sometimes referred to as "swelling agents" in that solvents swell the fiber to permit the dye molecules to be introduced into the fiber. It is believed that when the solvent is removed (e.g., by heating), the fiber returns to its original condition and the dye molecules are entrapped in the fiber. Certain of the organic solvent swelling agents, however, tend to damage the fibers by the swelling action, or result in the undesirable side effect of retention of solvent by the fibers. Retention of solvent can adversely affect both the appearance and the flame resistance of the fibers.
Retention of solvent is particularly frustrating when dyeing fibers such as the aromatic polyamides, i.e., poly-m-phenylene isophthalamide) ("Nomex.RTM.") and poly(p-phenylene terephthalamide) ("Kevlar.RTM."), polybenzimidazoles, aromatic polyimides, and the like which have excellent flame resistant properties but also are difficult to dye. In order to dye these fibers, some of the various organic solvents and carriers, e.g., DMAc, DMSO, DMF, acetophenone, etc., have to be used. Commercial use, however, may not be desired and may be perceived by some workers as noxious. Moreover, if non-water soluble, the solvents also tend to be retained by the fibers after dyeing. Thus, prior art techniques for dyeing these fibers can be counter-productive in that one of the problems caused by retention of solvent (e.g., decreased flame resistance) requires addition of a significantly large portion of a flame retardant to compensate for the reduction in the flame resistant properties of the fibers.
It is therefore an object of the present invention to provide an improved method for dyeing fibrous materials, and particularly to provide a method which obviates the need to utilize organic solvents and carriers.
It is another object of the present invention to provide a method having one or more particularly advantageous results, including increased efficiency in dye exhaustion and more uniform dyeing as compared to the absence of a carrier or organic solvent, and reduction of the retention of solvent as compared to organic solvents.
The improved method is particularly advantageous when used with difficult to dye fibers such as aromatic polyamide fibers (e.g., Nomex.RTM. and Kevlar.RTM. fibers), polybenzimidazole fibers, aromatic polyimide fibers, fibers of copolymers of the monomers thereof and blends thereof.